Appropriate valves are used in power hydraulics as shock valves. The valves safeguard the pressure between the Main Control Valve (MCV) and a power cylinder or hydro motor. Thus, even if the MCV is switched off, the respective cylinder is protected in the case of shocks acting on the devices or machines from the outside.
The valve can also be used to provide individual maximum pressure relief, which can be lower than the system pressure. For that reason, the valves are installed in pairs on the A and B sides of the directional control valves (MCV). If a valve responds to a shock load, a volume flow from the hydraulic cylinder (engine) is drained into the tank, and the cylinder evades the overload. To prevent cavitation in the hydraulic system and the connected valve and machine equipment, the opposing shock valve acts as a check valve and permits the oil to be sucked back out of the volume in the tank. In the state of the art (cf. the valve solution RD18329-32/11.10 by Bosch Rexroth Group published online in December 2014), direct-controlled installation kit valves are usually used.
In the case of these comparable valves, the valve seat of the check piston is incorporated in the control block. The seat of the pressure-limiting valve is located inside the movable check piston. If the seat of the check piston is damaged, the entire control block must be replaced, because the seat is part of the block. The pressure-limiting valve piston is relatively small in size because it is inside the check-valve piston. In this way, the flat pressure-limiting characteristic, which is desirable for the function, cannot be achieved. Owing to the design, implementing suitable valve damping is very difficult, resulting in a risk of instabilities in operating the valve.